The present invention relates to devices and methods that use electroencephalographic responses to auditory stimuli to evaluate hearing loss. More particularly, the present invention relates to devices and methods that use a subject""s evoked electroencephalographic response to an auditory stimulus to determine if a subject may have hearing loss, and that are capable of detecting problems with the connection between the subject and the hearing evaluation device.
In the past, hearing impairment in babies and children was often not detected until after it was observed that the baby or child did not respond normally to sound. Unfortunately, it often took months or even years for the parent to observe the impairment, and by that time the child""s language and learning abilities were negatively and often irreversibly impacted. Indeed, recent studies indicate that the vocabulary skills of hearing impaired children markedly increases if their hearing loss is detected early. The optimal time to evaluate hearing loss is thus immediately after birth, both because early detection allows for early treatment, and because parents often fail to bring their infants to later appointments. As a result, a number of states have implemented programs to evaluate newborns for hearing loss.
However, babies, especially newborns, cannot participate in traditional hearing tests, which require the subject to indicate if he or she hears the auditory stimulus. Thus, devices and methods have been developed to objectively determine hearing loss, without the voluntary participation of the subject. One such method involves analysis of the involuntary electroencephalographic (EEG) signals that are evoked from a subject in response to an auditory stimulus. It has been found that when a subject is able to perceive a sound having particular characteristics, a specific EEG waveform known as an Auditory Brainstem Response (ABR) is generated. This ABR response signal is typically small in magnitude in relation to general EEG activity. Therefore, statistical and signal processing techniques have been employed and developed to help detect, to a pre-defined level of statistical confidence, whether an ABR response has in fact been evoked. ABR testing is especially applicable to evaluation of infants, but can be applied to any subject.
The stimulus is applied through a transducer, which is connected to an earphone covering the subject""s ear. The earphone has a cavity that fits closely about the ear, and has an additional transducer, such as a microphone, that can detect ambient noise.
The ABR that is evoked in response to the auditory stimulus may be measured by use of surface electrodes on the scalp or neck. As a practical matter, the electrodes will also detect noise signals from neural activity (besides the ABR), muscle activity, and non-physiological environmental noise.
Thus, the device of the present invention is connected to the subject in two different ways. First, the earphone covers the ear, and creates a carefully defined acoustic environment through which the click stimulus is applied. Second, the electrodes detect the EEG signal of the subject.
There is potential for problems with both types of connections. If the earphone is incorrectly attached or deformed, the acoustic environment may be compromised and the accuracy of the testing can suffer. If the electrodes are reversed (i.e., the positive electrode placed where the negative should be and vice-versa), then the test data will be inverted, and the device will fail subjects who should pass.
Moreover, certain conditions can cause excessive impedance between electrodes, including certain skin conditions (such as excessively dry skin), or the detachment of the electrodes from the subject. Additionally, the wires connecting the electrode to the hearing evaluation device may become loose or detached. Excessive impedance, whatever its cause, can skew the data, and compromise or destroy the accuracy of the evaluation.
The present invention represents a major advance in the art because it provides means for evaluating the adequacy of the xe2x80x9cpatient connectionxe2x80x9d problems outlined above. The present invention continuously and automatically evaluates the acoustic environment created by the earphone, and notifies the operator of any inappropriate acoustic impedance resulting from detached or deformed earphones. It detects reversal of electrodes, and adjusts the hearing test accordingly. It also conducts electrode impedance and electrode xe2x80x9clift-offxe2x80x9d tests, and uses a new protocol under which an impedance test is automatically conducted if xe2x80x9clift-offxe2x80x9d is detected. The information provided by the impedance and xe2x80x9clift-offxe2x80x9d test can be used by the operator to re-secure the electrodes as necessary.
The present invention thus enables quicker and more trouble-free evaluation, because it detects patient connection problems before significant time has been wasted. The present invention also improves the accuracy of such evaluation.
The prior art does not continuously and automatically scan for inappropriate acoustic impedance caused by deformed or detached earphones, nor does it detect and account for electrode reversal. Moreover, the prior art does not provide for the improved electrode impedance and lift-off detection of the present invention.
The present invention provides a device and method for use in analyzing the EEG signal evoked in response to the auditory click stimulus, to determine if the subject suffers from hearing loss. Broadly, the invention is directed to devices and methods that are capable of evaluating and maintaining proper connections (through the electrodes and earphones) between the subject and the device.
In one embodiment of the invention, evoked EEG responses to auditory stimuli are collected, and organized into xe2x80x9csweeps,xe2x80x9d with each sweep containing the response signal for one auditory stimulus. The sweeps are organized into B blocks, with each block b containing a predetermined number of sweeps Nb.
The response signal for each sweep is digitized and converted into a series of binary numbers corresponding to whether the amplitude of the response signal is positive or negative at various points. in time. The digitized, binary waveform is compared to a benchmark ABR waveform to determine if the ABR is present. To make this determination, a polarity sum is calculated, which represents the sum of the polarities of the response signals within all blocks at each measured point in time. Statistical techniques are then used to determine if an ABR is present, relying upon the expected distribution of polarity sums in the absence of an ABR. A xe2x80x9cPassxe2x80x9d is triggered if the observed polarity distribution, as represented in a specifically defined test statistic, indicates that the likelihood that no ABR is present is below a predetermined threshold. After a certain predetermined number of blocks have been completed, evaluation. will cease if a xe2x80x9cPassxe2x80x9d has not yet been triggered. Evaluation may also stop if the hearing evaluation device determines that the subject is very unlikely to pass the test. If the evaluation ends without a determination that the subject has passed, the subject will be referred for further testing to determine if he or she in fact does suffer from hearing loss.
These evaluation procedures depend on the maintenance of accurate patient connections. If the electrodes are reversed, a subject with normal hearing may fail the test. If the earphone is improperly attached or deformed, then the proper acoustic environment will not be created, and the test data could be compromised. If there is too much impedance between the electrodes (potentially caused by oily skin, excessively dry skin, defective or worn electrodes, or detachment of the electrodes), then the device cannot collect meaningful data.
The present invention addresses these patient connection problems. Therefore, it is an object of the present invention to detect reversal of the electrodes. It is a further object of the present invention to detect if the improper placement of the earphones or their deformation prevents the formation of the proper acoustic environment. It is also an object of the present invention to conduct electrode impedance and lift-off detection, to continuously test for electrode lift-off, and to conduct an electrode impedance test during periods of frequent electrode lift-off.